From what I've read on the Net, each troop originated as a grenadier troop attached to the individual Household cavalry regiments, much the same way that the dragoon regiments each had a grenadier troop. And that by the time of the Seven Years War, they were detached, as 1st Troop, 2nd Troop, and so on of Horse Grenadier Guards. I also recall reading something along the lines that each troop's colors corresponded to those of the parent regiment. I've seen an image out there of the 2nd Troop's uniform, but so far, not the 1st.
This will be a big help, Martin. I have a Stadden 54mm Horse Grenadier, and I suspect that the casting is intended to represent a grenadier of the 1st Troop. That got me start on my search.
I've also pre-ordered Carl Franklin's book on the British Army uniforms from Amazon, and I think it covers all 4 Horse Grenadier troops.
Again, thanks very much, Martin, this is a breakthrough for me!
Prost!
Brad